DN0027
Design note
Setting Vout below the reference
using LC29300
Designs from our labs describe tested circuit designs from ST labs which provide optimized
solutions for specific applications. For more information or support, visit www.st.com
By Nicola Siciliano
Main components
LD29300
3A very low drop voltage regulators
TL431
Programmable voltage reference
Specification

Vin = 2.6V to 13V

Vout = 1.2V to 0.2V
Circuit description
Three pin adjustable voltage regulators don’t have a GND pin connection, and
the internal reference is floating (linked to Vout). For those types of voltage
regulators, the output voltage reaches the stable condition when the voltage
drop across the resistor, between output and adj pin, is equal to the internal
voltage reference.
Adjustable voltage regulators with a GND connection (4 pin or more) such as
the LD29300 have the internal reference referred to GND, and the output is set
when the voltage drop across the resistor between adj pin and GND is equal to
the internal voltage reference.
Figure 1 shows the two types of adjustable voltage regulators:


August 2014
one with reference to Vout
one with reference to GND
DN0027 Rev 1
1/4
www.st.com
Figure 1. A) adj with reference to Vout, B) adj with reference to GND
A) Adl Vreg with Vout reference
Vin
Vout
Vin Vout
Adj
Cin
B) Adj Vreg with GND reference
R2
Vin Vout
Vin
GND Adj
Vref
Cout
Cin
Vout
R2
R1
R1
Cout
Vref
Vout = Vref (1+R2/R1)
Vout = Vref (1+R1/R2)
When the internal reference is linked to GND, it is possible to set an output voltage lower
than the internal reference, using an additional voltage reference higher than the internal
reference.
Figure 2 shows this configuration. The resistor divider (R2 and R3) is now connected
between the external reference TL431, the adj pin (A) and the output (B). In a standard
configuration like in figure 1B, the current in the resistor divider flows from the output to
GND. While using the external reference, the current flows from the reference to Vout,
therefore the regulator drops the Vout (at point “B”) below the internal reference until the
voltage on the adjustable pin reaches the value of the internal reference.
Figure 2. Circuit for Vout lower the Vref
The TL431 reference is configured for 2.5V.
The Vout level is set through the following formula:
In the above formula, Iref is in the range of a few mA (2.375mA), 2.5V is the external
voltage reference and 1.25V is the voltage on the adj pin.
August 2014
DN0027 Rev 1
2/4
www.st.com
R4 is needed to provide a minimum load so that, in absence of load, the Iref current can
still flow to GND. To keep the system stable, the maximum value of R4 is given by the
equation.
Variations
The same technique can be used with any of the LDO regulators shown in Table 1
Table 1.
LDO
LDO regulators with reference to ground
Vin range
Iout
Vout accuracy
L6932
LD29080
LD29150
LD29300
LD39080
LD39150
LD39300
ST1L05
2V to 14V
2.5V to 13V
2.5V to 13V
2.5V to 13V
2.5V to 6V
2.5V to 6V
2.5V to 6V
3Vto 5.5V
2A
0.8A
1.5A
1.5A
0.8A
1.5A
1.5A
1.3A
±1%
±1%
±1%
±1%
±1.5%
±1.5%
±1.5%
±2%
Voltage Reference
Vref range
IK min (µA)
Precision
TL431
TL1431
TS2431
TS822
TS824-2.5
TS4040
2.5V - 36V
2.5V - 24V
2.5V - 24V
2.5V
2.5V
2.5V
1000
1000
1000
40
50
40
1%
0.25%
0.5
0.5%
0.5%
0.5%
Support material
Datasheet

LD29300: 3A very low drop voltage regulators

TL431: Programmable voltage reference
Application note

AN1761: TL431 accuracy of shunt voltage references in programmable mode
Revision history
Date
06-August-2014
August 2014
Version
1
Changes
Initial release
DN0027 Rev 1
3/4
www.st.com
IMPORTANT NOTICE – PLEASE READ CAREFULLY
STMicroelectronics NV and its subsidiaries (“ST”) reserve the right to make changes, corrections, enhancements,
modifications, and improvements to ST products and/or to this document at any time without notice. Purchasers should
obtain the latest relevant information on ST products before placing orders. ST products are sold pursuant to ST’s terms and
conditions of sale in place at the time of order acknowledgement.
Purchasers are solely responsible for the choice, selection, and use of ST products and ST assumes no liability for
application assistance or the design of Purchasers’ products.
No license, express or implied, to any intellectual property right is granted by ST herein.
Resale of ST products with provisions different from the information set forth herein shall void any warranty granted by ST for
such product.
ST and the ST logo are trademarks of ST. All other product or service names are the property of their respective owners.
Information in this document supersedes and replaces information previously supplied in any prior versions of this document.
© 2014 STMicroelectronics – All rights reserved
August 2014
DN0027 Rev 1
4/4
www.st.com